Highly non-linear metamaterials for laser technology

2014-07-03 – News from Physics Department

Non-linear optical materials are widely used in laser systems.
However, high light intensity and long propagation are required to produce strong non-linear optical effects.
Researchers at The University of Texas at Austin and the Technische Universität München
created metamaterials with a million times stronger non-linear optical response,
compared to the traditional non-linear materials,
and demonstrated frequency conversion in films 100 times thinner than human hair using light intensity comparable to that of a laser pointer.

The ultra-thin layers of the metamaterial were produced with this molecular beam epitaxy system.
– Photo: W. Hoffmann / TUM

Lasers have a fixed place in many fields of application. Yet, there are still
wavelengths for which either no systems exist, or at best only large and
expensive ones. On the other hand remote sensing and medical applications call
for compact laser systems, for example with wavelengths from the near infrared
to the terahertz region.

A team of researchers at the Technische Universität München (Germany) and the
University of Texas Austin (USA) has now developed a 400 nanometer thick
nonlinear mirror that reflects frequency-doubled output using input light
intensity as small as that of a laser pointer. For a given input intensity and
structure thickness, the new nonlinear metamaterials produce approximately one
million times higher intensity of frequency-doubled output, compared to the best
traditional nonlinear materials.

Furthermore, because the frequency conversion happens over subwavelength scales,
the demonstrated nonlinear mirrors are free from the stringent requirement of
matching the phase velocities of the input and output waves, which complicates
nonlinear optical experiments with bulk nonlinear crystals.

The new structures can be tailored to work at various frequencies from
near-infrared to mid-infrared to terahertz and can be designed to produce giant
nonlinear response for different nonlinear optical processes, such as second
harmonic, sum- and difference-frequency generation, as well a variety of
four-wave mixing processes.

The super sandwich

The magical material the physicists have created comprises a sequence of thin
layers made of indium, gallium and arsenic on the one hand and aluminum, indium
and arsenic on the other. They stacked about 100 of these layers, each between
one and twelve nanometers thick, on top of each other and sandwiched them
between a layer of gold at the bottom and a pattern of asymmetrical, crossed
gold nanostructures on top

Tuning the semiconductor layers thicknesses and the gold surface nanostructures
geometry, the researchers have two possibilities to adjust the structure to
resonate optimally with the desired wavelengths. For the initial demonstration,
the material converts light with a wavelength of 8000 nanometers to 4000
nanometers. “Laser light in this frequency range can be used in gas sensors for
environmental technology,” says Frederic Demmerle, project member at the Walter
Schottky Institute of the TU München.

400 nanometer thick nonlinear mirror that reflects frequency-doubled output using input light intensity as small as that of a laser pointer.
– Image: University of Texas (Austin)

Smaller than the wavelength

The ability to double the frequency of a beam of light stems from the engineered
electron states in the semiconductor material. When the semiconductor layers are
only a few nanometers thick, the electrons can only occupy specific energy
states and can be resonantly excited by the electromagnetic radiation.

“This kind of structure is called a coupled quantum well,” says Frederic
Demmerle. “Now, when we stack a further thin layer at a precisely defined
distance from the first layer, we can push these electron states closer together
or pull them apart, adjusting them precisely to the desired wavelength.”

Using the semiconductor material grown at TU München, a team of researchers at
the University of Texas, led by Prof. Mikhail Belkin and Prof. Andrea Alu,
designed a pattern of crossed gold structures tailored to have resonances at
particular input and output frequencies and fabricated then on top of the
semiconductor layer. It is this specific combination of semiconductor material
and gold nanostructures engineering that produces giant nonlinear response.

Although the patterns are considerably smaller than the wavelength of the
incoming light, the metallic structures ensure that the light is optimally
coupled to the material. Their special design also causes a strong increase in
field strength at specific locations, which further amplifies the nonlinear
response.

In the future, the team envisions using new materials realized along these lines
for other nonlinear effects. “Alongside frequency doubling, our structures may
be designed for sum- or difference-frequency generation,” says graduate student
Jongwon Lee, at the University of Texas, the lead author on the paper. “These
kinds of elements could be used to produce and detect terahertz radiation –
which is of interest for sensing and imaging applications, e.g., in medicine,
because it does not harm biological tissue.”

“This work opens a new paradigm in nonlinear optics by exploiting the unique
combination of exotic wave interaction in metamaterials and of quantum
engineering in semiconductors.” says Professor Andrea Alu.

The research was funded by the National Science Foundation of USA, the US Air
Force Office of Scientific Research, and the US Office of Naval Research, as
well as the German Research Foundation in the context of the Excellence
Initiative (Cluster of Excellence Nanosystems Initiative Munich, NIM).